This invention relates generally to tri-level imaging and more particularly to a method and apparatus for more efficiently transferring a tri-level image from a charge retentive surface to a substrate such as plain paper.
In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a charge retentive surface such as a photoreceptor. Only the imaging area of the photoreceptor is uniformly charged. The image area does not extend across the entire width of the photoreceptor. Accordingly, the edges of the photoreceptor are not charged. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, the image area contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-discharged (i.e. charged) areas of the photoreceptor while the other image areas correspond to discharged areas of the photoreceptor.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp, shown in FIG. 1a). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.dad or V.sub.c (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 v (V.sub.bb, shown in FIG. 1b) closer to V.sub.cad than V.sub.white (about 600 v), and the DAD developer system is biased about 100 v (V.sub.cb, shown in FIG. 1b) closer to V.sub.dad than V.sub.white (about 400 v).
As developed, the composite tri-level image initially consists of both positive and negative toners. To enable conventional corona transfer, it is necessary to first convert the entire image to the same polarity. This must be done without overcharging the toner that already has the correct polarity for transfer. If the amount of charge on the toner becomes excessive, normal transfer will be impaired and the coulomb forces may cause toner disturbances in the developed image. On the other hand, if the toner whose polarity is being reversed is not charged sufficiently its transfer efficiency will be poor and the transferred image will be unsatisfactory.
The amount of additional charge deposited on developed toner by a corona device, depends upon the toner's size, initial charge and polarity, and the amount of ac and dc corona current delivered to the region in the vicinity of the toner. To avoid overcharging the toner, a biased ac corona device is generally preferable to a dc device. The presence of both positive and negative ions in an ac corona discharge tends to equalize the charge among the toner particles due to the local electrostatic fields around the toner particles. In general, the change in the magnitude of the toner's charge for a given dc current into a region of the toner layer is also influenced by the magnitude of the ac current. If the toner layer is highly charged and the polarity of the dc component of the corona current flowing to the toner layer is the same as that of the toner, then the change in toner's charge for a fixed dc current will be smaller if an ac corotron is employed rather than a dc corotron.
Although there are ac corona current effects, it is the dc component of the corona current that is the dominant factor in determining how much net charge the toner receives in a given region. The dc current depends upon the ac current setpoint (for an ac corona device), the dc current setpoint, the potential at the toner layer surface prior to corona charging, and on the dielectric thickness of toner layer and photoconductor. Given the corona device characteristic, the dynamic dc current to a region moving past the charging device at a known speed can be modeled. However, here for purposes of illustration, it is sufficient to describe the dc current's behavior qualitatively.
The behavior of a corona device can be determined by measuring the current to a conductive plate as a function of the voltage applied to the plate (bare plate characteristics). In general, the bare plate characteristics (FIG. 3) for an ac corotron are such that the slope of the dc component of the current as a function of the plate potential is negative. As can been seen in FIG. 3, when the bare plate voltage increases in the negative direction, the negative dc current to the plate decreases (or the positivecurrent increases). This response is due simply to the change in the dc field between the corona wire and the bare plate. In a dynamic case, where a moving photoconductor with a developed toner on its surface is being charged, the situation is qualitatively similar to the bare plate case.
If an ac corotron is used to reverse the polarity of the negative toner in a discharged developed area of a tri-level image, disproportionatly more positive charge will be delivered to the toner that is already positive in the charged area developed parts of the composite image. This is just the opposite of what is desired because it makes it difficult to add enough charge to the negatively charged image parts to reverse their polarity without danger of overcharging the positively charged image parts.
It is well known in the prior art to subject a developed image on a charge retentive surface to corona discharge prior to image transfer for various reasons. For, example, U.S. Pat. No. 3,444,369 issued on May 13, 1969 relates to a method and apparatus for the reduction of background in transferred xerographic copy. A developed toner image on a photoconductive surface is subjected to a low level corona discharge of a polarity opposite the charge on the toner particles overlying the image areas. The corona discharge adjacent the image areas will be repelled by the like sign, but highly charged image areas of the photoconductive surface to thereby render the image area toner unaffected. The corona discharge adjacent the non-image areas of the photoconductive surface will not be repelled and will thus convert the toner overlying the non-image areas to a polarity opposite that on the image area toner particles. This will permit the electrostatic transfer of the image area toner, but will tend to suppress the transfer of the non-image area toner to a backing sheet.
It is also known to utilize light exposure and corona discharge prior to image transfer as shown in U.S. Pat. No. 4,506,971. In this device the light exposure occurs prior to the corona exposure. As stated therein, blurred images are minimized or eliminated in a xerographic reproduction prior to transfer by first exposing the image to light to at least substantially discharge the background around the image and to reduce the charge on the photoreceptor holding the image thereto. Secondly, a charge of opposite polarity of the charged photoreceptor is deposited onto the image and photoreceptor. This, as stated, produces a very stable image for transfer since a very strong holding force is produced to hold the image in place as the image enters the transfer station.
U.S. Pat. No. 3,784,300 issued on Jan. 8, 1974 relates to a copying apparatus with a pre-transfer station including a pre-transfer corotron and lamp arranged such that the light exposure of the photoreceptor is subsequent and not simultaneous with the pre-transfer corona charging.
U.S. Pat. No. 4,205,322 issued on May 27, 1980 relates to an electrostatic recording apparatus in which a toner image consisting of toner particles of at least two different kinds and of different polarities is efficiently and reliably transferred to a recording medium such as an ordinary sheet of paper. The toner particles having different polarities are all converted into those having one polarity and after such conversion the toner image (with its two kinds of particles) is electrostatically transferred to the recording medium, the transfer involving both kinds of particles at the same time.